PolyQuat is not only an algaecide, but is also a clarifier. It usually doesn't help as much with an algae bloom (especially in the beginning) mostly because it is simply overwhelmed with the quantity of algae. But as a clarifier, it helps clump suspended algae together so that it can filter out more effectively.

What is puzzling is that your pool was stuck in the blue-green phase and didn't continue to gray then milky then slightly cloudy then clear. This sounds like the algae was still growing almost as fast as it was getting killed by the chlorine. Perhaps it was on the "edge" of breaking (i.e. where chlorine kills algae faster than it can reproduce) and the PolyQuat pushed it over the edge, but this would mean the shock level wasn't high enough, which is very strange.

When you tested your water during the week, especially when your pool was "stuck" in the blue-green phase, did the FC level drop a lot overnight or was it fairly stable? This info might help us figure out what was going on so we can give better advice in the future.

Richard